This invention relates to a process for producing highly pure 3,3',4,4'-biphenyltetracarboxylic acid (hereinafter referred to as "BPTA") and highly pure 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter referred to as "BPDA"), which are starting materials for producing polyimide resins having excellent heat resistance.
BPDA has been noticed as a starting material for producing polyimide resins excellent in heat resistance. With an increase of demand for higher properties of polyimide resins, BPDA having higher purity has been demanded recently.
There have been proposed various processes for producing BPDA. One typical process comprises carrying out dehydrogenation dimerization of a dialkyl orthophthalate to produce 3,3',4,4'-biphenyltetracarboxylic acid tetraester, which is hydrolyzed to produce BPTA, followed by dehydration of BPTA. The purity of BPDA finally obtained is not always high due to contamination of a part of various impurities such as an unreacted starting material, byproducts, inorganic materials such as a catalyst and ions thereof.
Another process for producing BPTA comprises heating an aqueous solution of an alkali metal salt of 4-halogenoorthophthalic acid and an alkali hydroxide in the presence of a catalyst of metallic palladium carried on a carrier and methanol or the like as a catalyst to conduct dimerization reaction. After the reaction, the reaction solution is filtered to remove the palladium catalyst and the filtrate is added with an acid to deposit BPTA, followed by isolation of BPTA as crystals by heating and filtering with heating. The resulting BPTA is dried to give crude BPTA. The crude BPTA contains various impurities as mention above such as phthalic acid derived from the starting material, alkali metal salts, alkaline earth metal salts, and the like metal salts. There is also a process for producing BPTA by dimerization of dialkyl 4-halogenoorthophthalate in the presence of a nickel catalyst, and a co-catalyst. The purity of crude BPTA obtained in these processes is generally 85 to 90%.
Highly pure BPDA has been obtained by dehydration reaction of the crude BPTA, for example, by a dehydration process in the presence of an aliphthalic acid anhydride such as acetic anhydride, or by a process comprising subliming crude BPTA or crude BPDA with heating, followed by cooling the vapor thereof to give BPDA (Japanese Patent Unexamined Publication Nos. 61-249977 and 62-257934).
According to the former process, crude BPTA is refluxed with a large amount of an aliphthalic acid anhydride such as acetic anhydride and dehydrated to yield BPDA together with removal of impurities such as phthalic anhydride by dissolution due to a difference in the solubilities so as to increase the purity of BPDA. But this process is disadvantageous in that the handling is inconvenient due to the use of acetic anhydride, etc., having a strong irritating odor, and a production cost increases due to the consumption of a large amount of acetic anhydride, etc.
According to the latter process, the dehydration with heating and purification by sublimation are conducted. But this process is disadvantageous in that a production cost of BPDA increases due to the use of a large-scale apparatus and special control of conditions is necessary for the purification since phthalic acid, phthalic anhydride, etc. are also sublimed.